The present invention refers to a dishwashing machine provided with an improved arrangement for drying the washload in a system where the vapor released by the washload accommodated in a washing tank tends to condense mainly on the inner walls of the same washing tank. In other words, the present invention refers to a dishwashing machine which is adapted to dry the washload without using any special system provided with a condenser.
Dishwashing machines with washload drying systems provided with a condenser arrangement are largely known in the art. As disclosed for instance in U.S. Pat. No. 5,273 061, a specially provided condenser arranged in the washing tank of a dishwashing machine is traversed by a flow of cooling ambient air circulated therethrough by a fan. The vapor which is present in the washing tank at the end of a rinsing phase carried out with hot water, condenses on to the outer walls of the condenser, thereby promoting an efficient drying of the washload items.
Solutions are also well known, for instance from U.S. Pat. No. 5,056,543, in which the moisture-laden hot air present in the washing tank is circulated directly through the condenser (where it cools down and gives off its humidity) which can possibly be associated to auxiliary cooling means.
In any case, all such known drying arrangements making use of a condenser have a drawback in that the moisture-laden hot air contained in the washing tank of the machine can cool down to an excessive extent when in contact with the condenser, thereby cooling down the washload to be dried correspondingly. As a result, the evaporation of the water from the washload is reduced, so that it becomes necessary for the washload to be submitted to forced heating, which is usually performed by appropriately operating the heating elements of the machine "in air", i.e., under dry conditions, when they are not wetted by the washing or rinsing liquor. As commonly known, this gives rise to undesired effects of possible overheating of component parts of the machine and plates additional stresses onto the electromechanical and/or electronic interfaces associated to the heating elements themselves. This adds to the fact that the provision of a condenser and the component parts that are usually associated therewith undesirably complicates the structure of the whole dishwashing machine and substantially increases the manufacturing and operating costs thereof.
On the other hand, in the dishwashing machines of the traditional type that are not equipped with a condenser for the drying operation, the vapor that is produced inside the washing tank at the end of a rinsing phase carried out with hot water generally gives rise an undesired thermal stratification. In particular, the temperature of such a vapor is higher in the upper portion of the tank interior (where at least a first washload holding rack or basket is usually provided), while it is lower in the lower portion of the same tank interior, where at least a second washload holding rack is usually provided. The temperature of the washload items in the different holding racks will of course be correspondingly different. For instance, approximately 10 minutes from the conclusion of a rinse carried out with water which had been heated up to 65.degree. C., in correspondence of the ceiling of the washing tank, to a minimum of approximately 25.degree. C. in correspondence of the bottom of the same washing tank.
It is substantially in these conditions that the washload drying process takes place owing to the evaporation of residual water from the same washload items during a subsequent pause in the operation of the machine. The vapor released by the washload items tends to condensate mainly on the inner walls of the washing tank, which during this period of time tend to cool down, in a substantially homogeneous manner, more rapidly than the washload items themselves. Since the extent to which the washload items are able to dry is proportional to the thermal difference existing between the washload items themselves and the walls of the washing tank, the result is that the (colder) washload items arranged in the lower rack get dry in a less efficient manner than the warmer washload items arranged in the upper rack. In practice, the extent to which the washload items in the lower rack get dry is sometimes quite unsatisfactory. Anyway, the overall drying effect of the washload items in the washing tank of the machine appears to be undesirably non-homogeneous.
It is therefore a common practice to try to improve the drying effect of the washload items in a dishwashing machine by providing, as this is for instance described in the Italian Utility Model Application No. PN91 U 000045, means that are adapted to bring about, when the machine is not operating, a natural circulation of ambient air through the washing tank of the dishwashing machine. Such a solution is, however, associated with an important drawback in that the dishwashing machine is able to release vapor into the surrounding ambient atmosphere, thereby giving rise to a number of practical problems as anyone skilled in the art is well aware of.
France Patent No. FR-A-1 116 585 discloses a dishwasher with a wash tub in which a propeller is provided to project water onto the crockery for washing purposes. To dry the crockery, the propeller is subsequently operated to agitate hot air inside the wash tub so that vapor released by the crockery is brought into contact with the walls of the wash tub, where it is condensed. Thermal stratification is, however not prevented in an effective way in the whole wash tub and, in fact, the walls of the tub itself must not be thermally isolated in order to be kept sufficiently cold, thereby enabling condensation of vapor thereon. In other words, the cold walls of the wash tub operate as a condenser. As a consequence, unacceptable thermal losses occur through the walls of the wash tub during the wash cycle.